Prior fasteners of the corrugated type have employed intermediate sections to accomplish the joining of two pieces of material. Cary U.S. Pat. No. 1,328,911 issued June 22, 1920 is illustrative.
Prior patents have also suggested curved center sections (see Johnson U.S. Pat. No. 1,876,683 issued Sept. 13, 1932 and Dimoush U.S. Pat. No. 2,156,682 issued May 2, 1939) but none have shown a curved center section which is shaped, sized and operable as taught by the present invention.
Fastener strips such as those disclosed in Cusumano U.S. Pat. No. 2,942,267 have 15% to 30% of the surface of the fastener adhesively secured to the adjacent fastener requiring a large force to cleanly shear the leading fastener from the strip. The fastner strip of the present invention has only about 2% of the surface of the fastener adhesively secured to the adjacent fastener, yet, the resulting fastener strip is sufficiently functionally strong to withstand handling, vibration and recoil of a driving tool, while offering a substantially reduced force or resistance to the shearing of the leading fastener from the strip.
It has been determined that the integrity of a joint held together by a corrugated fastener depends primarily on three factors:
(1) design of the inclined ridges which effect pulling together of the joined pieces as the corrugated fastener is driven,
(2) design of the center section, and the
(3) quality and location of the bond between the surface of the corrugated fastener and the surface of the fastened pieces.
In corrugated fasteners having uniform corrugations or flutes, the integrity of the joint is only as good as the forces required to straighten the flute of the corrugated fastener positioned at the abutting surfaces of the two pieces of material.
Fasteners with straight intermediate sections such as Cary U.S. Pat. No. 1,328,911 have the tendency to tear the pieces being joined causing loosening of the joint. An intermediate section which is straight before driving cannot further tension or absorb energy during driving and create a joint tensioned by the driven fastener.